Apparent @"count:" issue on iPod's with iPhone OS 3.1.3

I have tracked down a crash users of my app on iPod Touches with iPhone OS 3.1.3 have been having: for some reason, using the "@count:" expression on the CoreData store is causing a crash. Here's the relevant code:

NSExpression *avgExpression = [NSExpression expressionForFunction:@"average:" arguments:[NSArray arrayWithObject:keyPathExpression]];
NSExpressionDescription *ed = [[NSExpressionDescription alloc] init];
[ed setName:@"avgScalar"];
[ed setExpression:avgExpression];
[ed setExpressionResultType:NSFloatAttributeType];

NSExpression *countExpression = [NSExpression expressionForFunction:@"count:" arguments:[NSArray arrayWithObject:keyPathExpression]];
NSExpressionDescription *ed2 = [[NSExpressionDescription alloc] init];
[ed2 setName:@"countScalar"];
[ed2 set开发者_运维百科Expression:countExpression];
[ed2 setExpressionResultType:NSInteger16AttributeType];

[request setPropertiesToFetch:[NSArray arrayWithObjects:ed,ed2,nil]];

When the fetch is performed, I get the following error with the crash:

* Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '* -constantValue only defined for abstract class. Define -[NSKeyPathExpression constantValue]!'

If I restrict the fetch to the avgExpression (@"average:"), all works fine. There's something wonky about the @"count:" expression, so far as I can tell only on iPod Touches with iPhone OS 3.1.3. Any ideas?

I could alternately perform a full fetch and use key path operators to get the averages and counts I need. However, I worry this approach would be much more expensive. Anyone know if it is?

---

Here's the best solution for the older OS that I could find, taking only a small performance hit.

1. Execute my fetch request for avgExpression only

2. Clear out the expression part of the request:

    [request setPropertiesToFetch:nil];
   
    [request setResultType:NSManagedObjectResultType];
   

3. Perform a count-only fetch:

    [managedObjectContext countForFetchRequest:request error:&error];
   

It's clear from working through this issue that the internals of CoreData changed from iOS 3 to iOS 4. You can see this, for example, by NSLog'ing a property-focused NSFetchRequest object in both OS's.